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CHAP. XXL THE SIMPLEST ELEMENTS APPEAR FIRST. 



WITH regard to the substances which appear in the hottest stars, 

 the all important, the first point to make, is that the chemical forms 

 we see are amongst the simplest. 



How can this be determined 1 In two ways. The chemist will 

 acknowledge that an element of low atomic weight is simpler, that is, 

 has less mass than an element of high atomic weight. If we rely upon 

 spectrum analysis we can say, when dealing with the question of 

 *' series," that the elements which most readily give complete series 

 are in all probability simpler than those which give none, and this is 

 still truer when we find that all the lines in the spectrum of a sub- 

 stance can be included in those rhythmical series, as happens in the 

 case of hydrogen and the cleveite gases. Judged then by these 

 standards it is certain that the first stage of inorganic evolution, if 

 there has been such an evolution, is certainly a stage of simplest forms 

 as in organic evolution, whatever view we take of the nature of the 

 " atom." 



It is worth while to compare in detail the results obtained by this 

 newest form of spectrum analysis relating to "series," with the 

 earliest stellar forms, because it is evident that we are here in presence 

 of the beginning of a new method of study of the nature of the 

 so-called chemical elements. 



We found that the hottest stars contained hydrogen, helium and 

 asterium. We have also found (Chap. X) that those substances 

 have the simplest series ; that is to say, one set of three. It is more 

 than probable, although it is not absolutely established, that the 

 lithium group of metals is also represented in stars of very high 

 temperature. There, again, we have the simple series of one set of 

 three. About sulphur we do not yet know positively, but it is 

 probable, I think, that sulphur may exist in the hot stars. There, 

 again, we get another simple set of three ; so that for three perfectly 

 certain constituents of the hottest stars, together with one present in 

 all probability and one doubtful, we are dealing with the simplest 

 series. 



But now comes the remarkable fact that side by side with these 

 simple substances we get in the hottest stars magnesium, calcium and 

 silicium. Of the " series " conditions of the last we know nothing. Of 

 magnesium and calcium only subordinate series have been determined. 



